Declarations

While syntactically similar, these two declarations do fundamentally different
things.

Validations are behavior

The validates_presence_of :title declaration changes the behavior of
the save method (and other methods that use save), and should be specified
explicitly. Here’s an example using shoulda matchers:

validate_presence_of_title.rb

123

describeArticledoit{shouldvalidate_presence_of(:title)}end

Even though the matcher’s name looks just like the likely implementation, the
validate_presence_of matcher specifies that you can not save an Article
without a non-nil value for title, not that the
validates_presence_of(:title) declaration exists.

Associations are structure

The has_many declaration exposes a comments method to clients that appears
to be a collection of Comment objects. Doing Test-Driven Development, you
would add this declaration when a specified behavior requires it e.g.

with_comments_by.rb

123456789

describeArticledodescribe"#with_comments_by"doit"finds articles with comments by the submitted comment_author"doarticle=Factory(:article)article.comments<<Factory.build(:comment,:author=>"jdoe")Article.with_comments_by("jdoe").shouldeq([article])endendend

This example needs a comments method that returns a collection in order to
pass. If it doesn’t exist already (because no other example drove you to add
it), this would be all the motivation you need to introduce it. You don’t need
an example that says it "should have_many(:comments)".

Testing the framework

Some will argue that we don’t need to spec validations either, suggesting that
it "should validate_presence_of(:title)" is testing the Rails framework,
which we trust is already tested. If you think of TDD as a combination of
specification, documentation, and regression testing, then this argument falls
short on the specification/documentation front because the validation is
behavior and, thus, the spec should specify the validation.

Even if you view testing as nothing more than a safety net against regressions,
the argument still falls down in the face of refactoring. If we add a Review
class that also has_many(:comments) and validates_presence_of(:title), and
we want to extract that behavior to a Postable module that gets included in
both Article and Review, we’d want a regression test to fail if we failed
to include either of those declarations in the Postable module.

But declarations are already declarative!

Another argument is that declarations supply sufficient documentation. e.g. we
can look at rental_contract.rb and know that it validates the presence of
:rentable:

This is an interesting argument that I think has some merit, but I think it
would require an extraordinarily disciplined and consistent approach of using
declarations 100% of the time in model files such that each one is the spec
for that model, e.g.

100% may sound extreme, but as soon as we define a single method body in any
one of the models, the declarative nature of the file begins to degrade, and so
does its fitness for the purpose of specification. Plus, if we can only
understand the expected behavior of a model by looking at both its spec and
its implementation, we’ve lost some of the power of a test-driven approach.

What do you think?

Do you spec associations? If so, what value do you get from doing so? If not,
have you run into situations where you wished you had?